1. Field of the Invention
The present invention relates generally to a camera shutter having a shutter blade that also functions as a diaphragm for defining an effective diameter of a lens aperture.
2. Description of Related Art
Conventionally, when taking a picture of an object by use of a still camera, a photographer sometimes wants to optimize a combination of a lens aperture size and a shutter speed according to a picture shot situation. For such a purpose, a camera system is available in which a diaphragm aperture plate for defining the effective diameter of the lens aperture is arranged separately from a shutter blade for opening and closing the lens aperture. In such a camera system, the diaphragm and the shutter blade are mounted individually. Even a camera having one or two steps of aperture stops in a simple manner uses the above-mentioned construction, if a certain aperture size accuracy is required. Especially, in exposure control of a digital still camera using a CCD (Charge Coupled Device), the number of aperture steps may be small but high aperture size accuracy and high-speed shutter blade operation are required. This is because CCD-based digital cameras have a narrow latitude.
However, the above-mentioned conventional camera system requires more than one actuator for driving the diaphragm aperture plate and the shutter blade separately from each other, thereby causing serious limitations in terms of manufacture cost and space efficiency. For an alternative means, a structure is known in which the shutter blade also functions as the diaphragm. In this structure, the shutter blade is stopped halfway between an open position and a closed position to obtain a predetermined diaphragm aperture size. To do this, the shutter blade must be stopped halfway by means of a mechanical stopper, and then this mechanical stopper must be released to close the shutter blade. This inevitably complicates the configuration of the mechanism of the camera.
It is therefore an object of the present invention to provide a camera shutter apparatus for solving the above-mentioned problems.
In carrying out the invention, there is provided a camera shutter apparatus basically comprising a shutter blade, a setting member, a drive source, and a controller. The shutter blade is adapted to move in open and close directions relative to a lens aperture from a home position to open and close the lens aperture for an exposure operation. The setting member can hold the shutter blade with a biasing force at a stop position corresponding to a predetermined diameter of the lens aperture for setting the lens aperture with the predetermined diameter. The drive source generates a variable driving force necessary for moving the shutter blade. When moving the shutter blade from the home position to the stop position so as to set the lens aperture and then moving the shutter blade in the close direction for an exposure operation, the controller controls the drive source to generate a driving force sufficient for starting or activating the shutter blade from the home position and smaller than the biasing force of the setting member, thereby holding the shutter blade at the stop position in abutment against the setting member. Preferably, in order to set the lens aperture with different diameters, at least two setting members are arranged which can selectively hold the shutter blade at different stop positions having different diameters. Preferably further, a pair of shutter blades are arranged and adapted to move in linked association with each other. One of these shutter blades abuts with one of the two setting members to set the lens aperture at a first diameter, while the other abuts with the other setting member to set the lens aperture at a second diameter. Preferably still, at least two shutter blades are arranged and adapted to move in linked association with each other, and an auxiliary setting member is arranged which imparts an biasing force smaller than that of the setting member. In such a case, one of the shutter blades abuts against the setting member to be held to the stop position, and the other of the shutter blades abuts against the auxiliary setting member to suppress an unnecessary play of the other shutter blade. In one form, the shutter blade is held at the stop position after the shutter blade is started in the close direction for setting the lens aperture from the home position at which the lens aperture is fully open, and the controller controls the drive source to raise the driving force to move the shutter blade forward from the stop position in the close direction for fully closing the lens aperture. In another form, the shutter blade is held at the stop position after the shutter blade is started in the open direction for setting the lens aperture from the home position at which the lens aperture is fully closed, and the controller controls the drive source to reverse the driving force to move the shutter blade backward from the stop position in the close direction for fully closing the lens aperture.
According to the invention, the shutter blades are made to function also as a diaphragm, thereby eliminating use of an additional drive motor conventionally required for driving a diaphragm aperture plate. To hold the shutter blades to the stop position for setting the effective aperture diameter, the output of the drive source is made greater than an initial shutter blade driving torque and smaller than the biasing force of the setting member. When moving the shutter blades from the stop position to the fully closed position, a drive force greater than the biasing force of the setting member is applied to the shutter blades. For example, when moving the shutter blades from the open home position into the close direction, the output of the drive source is regulated smaller than the biasing force of the setting member, hence the movement of the shutter blades into the close direction is restricted or blocked by the setting member and is held at the stop position corresponding to the desired aperture diameter. After passing of a predetermined time, increasing the output of the drive source over the biasing force moves the shutter blades from the stop position to the fully closed position. Thus, electrical controlling of the drive source can set the shutter blades at a desired stop position corresponding to a desired aperture diameter, thereby eliminating use of conventional complicated mechanical stopper structures.